In known wireless cellular telecommunications networks, an area covered by the network is divided into a plurality of cells. Each of these cells has a base station which is arranged to transmit signals to and receive signals from mobile stations located in the cell associated with the respective base station. Mobile stations will be in active communication with the base station associated with the cell in which the mobile station is located.
Each base transceiver station is, in an example of the GSM (Global System for Mobile Communications) standard, arranged to receive N frequency channels out of a possible 125 frequency channels F1 . . . F125 available. The total bandwidth occupied by the 125 frequency channels is 25 MHz which means that each frequency channel has a bandwidth of 200 kHz. Other bandwidths between 4.8 and 30 MHz exist. Each channel is divided into frames each of which are further sub-divided into time slots. The GSM standard is a time division multiple access (TDMA) system and accordingly different mobile stations will be allocated different time slots. The base transceiver station will therefore receive signals from different mobile stations in different time slots using the same frequency channel. N is usually less than 125. Each base station uses different frequencies to transmit to the mobile stations. Again 125 channels are available for the transmission of signals by the base station. N channels are also used out of the available 125 channels for transmission by the base station. The frequency used to transmit to a mobile station is generally separated by a constant value from the frequency used by the mobile station to transmit to the base station. In the following, reference is being made to traffic channels.
As a cell is served by a specific base station, all mobile stations within the cell transmit and receive signals at frequencies which have been allocated to the associated base station. A set of N transmit frequencies and a set of N receive frequencies are allocated to base stations and mobile stations.
Each set of frequencies used in a particular cell need to be different from the set of frequencies used in an adjacent or neighboring cell. This is to avoid co-channel interference. For example, if a first cell used a frequency set comprising frequencies F1, F4, F7 then all adjacent cells would use frequencies other than F1, F4, F7.
With GSM there are 125 frequency channels available, as mentioned previously, which means that there will need to be the same frequency channel allocated to more than one cell within the network. The cells with identical frequency sets could be a large distance apart, for example in the countryside where there tends to be fewer mobile stations simultaneously requesting service. Conversely, the cells with identical frequency sets could be a very small distance apart, for example in a major city centre where there tends to be a large number of mobile stations simultaneously requesting service. The neighboring cells which are allocated different frequency channel sets are arranged together in clusters.
In areas where there is a large number of mobile users all trying to communicate with the base station at the same time, for example in major conurbations, there is a need for a correspondingly large number of frequencies to be allocated to each cell, in other words there is a need for a large frequency channel set to be associated with each heavily used cell. This is because many mobile users require service from each base station at substantially the same time. Accordingly frequency reuse patterns are much tighter. That is cells using the same frequency are relatively close together. Additionally, channels of a similar frequency are likely to be present in adjacent cells leading to adjacent channel interference.
As the number of mobile users increases and the size of cells have been made smaller, adjacent channel interference and co channel interference has began to becomes more problematic. This is especially the case where the network is synchronized and cyclic frequency hopping is in use. This is because one user will be effected by the same interferer at all times whereas if synchronization and cyclic frequency hopping were not in use, the interferer would be randomly spread amongst all the users.
To mitigate co channel interference, the networks are currently designed to have sparse reuse of frequency. This means that the distance between cells which are allocated the same frequency channels are relatively long. This reduces interference within the network and also means that adjacent channel interference is not generally problematic. However this does not allow the full capacity of networks to be realised and as the number of subscribers expands and the size of cell reduces, this strategy is disadvantageous.